The invention relates to a process and apparatus for machine sorting of sawn timber according to strength.
In one known process of this type (GB 2105856A) the test roller 9 is set by manual operation of a wheel 15 in dependence upon the type of sawn timber L to be sorted, and that operating position is then not changed thereafter. The shaft of the test roller 9 is mounted on a stable arm 13 which itself is pivotally mounted on a shaft 14 of the machine frame 10. The natural curvature of the wood is not measured. Instead, each piece of timber is turned through 180.degree. about its longitudinal axis after the first pass, is transported back to the input of the machine by taking it back through the machine in the opposite direction, and is sent through the machine again in a second pass. This is time-consuming and operationally costly and can lead to an overstressing of the wood if the wood has natural curvature. Also, the rotary drives of the driven clamping rollers 2, 4 must be reversible.
From GB 1104986 is known a process in which only one pair of rotatably drivable rollers 2, 4 is provided, one of which rollers 4 can be transversely biassed 5. A stationary support roller 3 is positioned at the side of the stationary roller 2 and is spaced from it. At the side of the biassable roller 4, between the roller pair 2, 4 and the support roller 3, is a test roller 7 which is arranged to be urged with constant force (page 2, line 115 ) against the sawn timber by means of a drive 8. The effective bending deflection of the timber resulting from this is detected by a sensing element 9 which is in contact with the opposite side of the timber and is converted into appropriate electrical signals by means of micro switches 11, with these signals being passed to an evaluation circuit. The effective bending deflection can be falsified by a natural curvature of the timber. Consequently, in advance of the roller pair 2, 4 a test arm 15 is arranged which is pivotable about the axis of the stationary roller 2. In advance of the roller 2 is arranged a roller 13, stationary on the test arm 15, at the same spacing from the roller 2 as there is between the rollers 2 and 3. The roller 13 is maintained in permanent contact with the wood by means of a biassed 18 counter-roller 14. At the same geometrical position as the sensing element 9 relative to the rollers 2, 3 is arranged a sensing element held by an arm 16 of the test arm 15 for sensing the natural curvature of the timber relative to the rollers 2, 13. The oscillations of the arm 16 relative to the test arm 15 are converted 19 into electrical signals which correspond to the natural curvature. In the evaluation circuit the computation of the natural curvature is effected by the effective bending deflection of the timber measured by the sensing system 9 to 11. The constant pressure force of the test roller 7 does not take into account any changing modulus of elasticity of the timber.
From the journal "Microtecnic" No. 1, 1991, Zurich, Switzerland, pages 10 to 13, Hofmann "PSD Elements Broaden the Optical Measuring Techniques", various optical distance sensors are known for contactless distant measurement.
From the journal "Holz-Zentralblatt" No. 97, 14th Aug. 1987, Stuttgart, Germany, page 1360, Tebbe "The Current State of Machine Quality Sorting", column 3, lines 31 to 53; FIG. 4, there is known a decisive improvement in the sorting using a combination of sorting parameters. It makes possible the combination of modulus of elasticity and knottiness. The knottiness could be determined for example by an irradiation method (microwaves or X-rays).